Manganese dioxide is commonly employed as a cathode active material in commercial batteries including heavy duty, alkaline and lithium cells. Battery grade manganese dioxide has been derived from naturally occurring manganese dioxide (NMD) and synthetically produced manganese dioxide. Synthetic manganese dioxide is basically divided into two categories: electrolytic manganese dioxide (EMD) and chemical manganese dioxide (CMD). NMD because of its high impurity content cannot be employed in alkaline or lithium cells.
EMD (electrolytic manganese dioxide) has become the preferred form of manganese dioxide for use in Zinc/MnO2 alkaline or lithium cells. EMD (electrolytic manganese dioxide) can be manufactured from the direct electrolysis of an aqueous bath of manganese sulfate and sulfuric acid. The EMD is a high purity, high density, gamma manganese dioxide, desirable as a cathode material for electrochemical cells particularly Zn/MnO2 alkaline cells, Zn-carbon and lithium/MnO2 cells. During the electrolysis process the gamma EMD is deposited directly on the anode immersed in the electrolysis bath. The anode is typically made of titanium, lead, lead alloy, or graphite. The EMD is removed from the anode, crushed, ground, washed in water, neutralized by washing with dilute NaOH, Na2CO3, NH4OH or LiOH, and dried in a rotary dryer. The EMD product can then be used as cathode active material in an alkaline cell, typically a zinc/MnO2 alkaline cell. The EMD product is generally heat treated to remove residual water before it can be used in a lithium cell. Conventional electrolysis processes for the manufacture of EMD and a description of its properties appear in Batteries, edited by Karl V. Kordesch, Marcel Dekker, Inc. New York, Vol. 1 (1974), p. 433-488. Conventional electrolysis processes for production of MnO2 are normally carried out at temperature between about 80 and 98° C.
M. Mauthoor, A. W. Bryson, and F. K. Crudwell, Progress in Batteries & Battery Materials, Vol. 16 (1997), pp. 105-110 discloses an electrolysis method for manufacture of manganese dioxide (EMD). The electrolysis is performed at temperatures between 90 and 108° C. Although Mauthoor reports that discharge capacities of MnO2 synthesized by electrolysis of an aqueous bath of MnSO4 and H2SO4 at between 95° C. to 108° C. was about 9% higher than that for MnO2 material produced at 95° C., there was no substantial difference among the three MnO2 products produced at electrolysis temperatures of 100° C., 105° C., and 108° C. In fact, as Mauthoor increased the electrolysis temperature from 105 to 108° C., the percent MnO2 in the electrolysis product and the discharge capacity of the MnO2 product both decreased slightly. Thus, electrolysis at temperatures higher than 108° C. were not attempted or contemplated.
M. Ghaemi, Z. Biglari, and L. Binder, Journal of Power Sources, Vol. 102 (2001), pp. 29-34 discloses effects of varying temperature of the electrolysis bath during manufacture of the manganese dioxide (EMD). Specifically the properties of the EMD product were investigated when the EMD was employed in a rechargeable alkaline cell. The electrolysis bath temperatures were varied in a range between 60° C. and 120° C. The data is oriented towards the performance of the rechargeable cell with no data specifically dealing with performance of the EMD in a primary cell. Also, when the rechargeabe cells were tested the first cycle performance data did not show any improvement with cathodes of EMD produced under the higher electrolysis bath temperatures, e.g. 115-120° C. compared to conventional bath temperatures, e.g. 80-98° C.
In commercial EMD production, the electrolysis is normally carried out at temperatures between 94° C. and 97° C. and at current densities between 2 and 10 Amp/ft2, more typically between 4 and 10 Amp/ft2 of anode surface area. A titanium anode and graphite or copper cathode are typically employed. Increasing current density tends to increase the MnO2 specific surface area (SSA). When electrolysis is carried out at conventional temperatures and current density is increased beyond the normal bounds, there is a tendency for the specific surface area (SSA) of the MnO2 product to increase to a level which is outside (greater than) the desired range of between 18-45 m2/g. Thus, at conventional temperatures it is very difficult to increase the current density and the deposition rate above a level of between about 10 to 11 Amp/ft2 (108 to 119 Amp m2) without adversely affecting the quality of the product.
In addition, under conventional conditions of temperature and electrolyte composition, at current densities greater than 10 Amp/ft2 (108 Amp/m2) there is a tendency for passivation of the titanium anode to occur after a period of time, which may be shorter than the normal plating cycle of 1.5 to 3 weeks. The higher the current density, e.g. 12 Amp/ft2 (130 Amp/m2) rather than 10 Amp/ft2 (108 Amp/m2), the sooner such passivation is likely to occur. Passivation involves the formation of an insulating oxide film on the surface of the titanium, resulting in an increase in the operating Voltage of the anode. Once started the problem is self accelerating and soon results in a precipitous Voltage rise which exceeds the capability of the power supply followed by a loss of current, ending in complete and irreversible shut-down of the plating process. Often a number of anodes will fail simultaneously due to passivation. When this occurs, the anodes must be withdrawn, deposited EMD removed and the anodes must be surface treated to remove the tenacious oxide film prior to being returned to service. This is a highly disruptive and expensive problem. In a commercial setting, great care is taken to avoid anode passivation and a margin of safety is preserved in setting the current density below that which borders on passivation, EMD quality considerations aside.
V. K. Nartey, L. Binder, and A. Huber, Journal of Power Sources, Vol. 87 (2000), p. 205-211 describes an electrolysis process for making MnO2 wherein the electrolysis bath was doped with TiOSO4. The MnO2 was used in an alkaline rechargeable battery. The reference states at page 210, col. 1 that the MnO2 with TiOSO4 doping (called M2, Table 7) performed poorly on the initial discharge cycle (i.e. similar to a primary, non-rechargeable cell) despite a high specific surface area. When the bath was doped with TiO2 the MnO2 product (called M1, Table 7) performed better on the initial discharge cycle, but still did not perform as well as the control MnO2 (commercial grade EMD Tosoh GH-S). The electrolysis bath for the experiments described in Huber, et al. was maintained at conventional temperature of 98° C. and was performed at conventional current density of 6 milliAmp/cm2 (5.57 Amp/ft2) based on anode surface area.
Conventional battery grade manganese dioxide does not have a true stoichiometric formula MnO2, but is better represented by the formula MnOX, wherein x is typically between about 1.92 to 1.96, corresponding to a manganese valence of between about 3.84 and 3.92. Conventional EMD may typically have a value for x of about 1.95 or 1.96, corresponding to a manganese valence of 3.90 and 3.92, respectively. In addition to manganese (Mn) and oxygen (O), conventional electrolytic manganese dioxide (EMD) also contains a certain quantity of SO4= ions and of H+ ions (protons) in the crystal lattice. When heated to temperatures above 110 deg. C., the lattice protons combine with oxygen and are liberated as H2O. Conventional EMD also has a real density of between about 4.4 and 4.6 g/cm3.
CMD has for many years been economically produced commercially, but such commercial chemical processes while yielding high purity MnO2, do not yield densities of MnO2 comparable to that of EMD. As a result EMD has become the most widely used form of battery grade MnO2, particularly for alkaline and lithium cells, since in such application it has become most desirable to employ high density MnO2 to increase the capacity of these cells. However, in the course of conventional manufacture of EMD, it has been difficult to significantly alter important properties, such as surface area and activity, without adversely affecting the density.
U.S. Pat. No. 2,956,860 (Welsh) discloses a chemical process for the manufacture of battery grade MnO2 by employing the reaction mixture of MnSO4 and an alkali metal chlorate, preferably NaClO3. This process is known in the art as the “Sedema process” for manufacture of chemical manganese dioxide (CMD). The reaction is carried out in the presence of solid MnO2 particles which act as a catalyst and nucleation site for deposition of the MnO2 formed from the reaction of MnSO4 and alkali metal chlorate. As the reaction proceeds, MnO2 which is formed precipitates onto, and even into, the MnO2 substrate particles. The resulting MnO2 product from the Sedema process takes the form of smooth-surfaced spherical particles. However, the MnO2 does not have a density as high as that obtained in EMD. Significantly higher densities of the MnO2 product are not obtainable by controlling reaction rate with alkali metal chlorate. Also the MnO2 produced from the process disclosed in this reference cannot be readily deposited on substrates other than manganese oxides. If an alternative substrate or no substrate is employed, the MnO2 product precipitates out during formation as a light fluffy product which is unacceptable as battery grade MnO2.
An article by K. Yamamura et. al., (“A New Chemical Manganese Dioxide for Dry Batteries,” Progress in Batteries & Battery Materials, Vol. 10 (1991), p. 56-75) discloses another process for manufacturing gamma MnO2. The process referenced as the “CELLMAX” (CMD-U) process involves special treatment of purified crystalline MnSO4 to produce an electrochemically active high density gamma MnO2. The product has a surface area and particle appearance similar to electrolytic manganese dioxide (EMD), but differs in its pore size, tap density and particle size distribution. The process consists of the steps of leaching manganese ore, crystallizing, adjusting the pH, compressing and grinding. In the process the manganese sulfate solution extracted from the manganese ore is purified, crystallized under optimum conditions and roasted at very high temperature. The product Mn3O4 is oxidized to Mn2O3 by oxygen at high temperature. The Mn2O3 is subjected to acid treatment to yield gamma MnO2 which in turn is compressed to yield a higher density. Although a high density gamma MnO2 product is reported, the process has the disadvantage of involving a number of reaction and processing steps which require careful control and would be expensive to implement.
There are increasing commercial demands to make primary alkaline cells better suited for high power application. Modern electronic devices such as cellular phones, digital cameras, toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Such devices demand high power, for example, an AA cell may be required to deliver high power between about 0.5 and 2 Watt which corresponds to current drain rates between about 0.5 and 2 Amp, more usually between about 0.5 and 1.5 Amp. Thus, it is desirable to provide a way of reliably increasing the useful service life of conventional primary alkaline cells particularly for cells to be used in high power applications, without adversely affecting cell performance on medium or low power applications.
Accordingly it is desirable to produce an improved form of manganese dioxide which extends the useful service life of electrochemical cells, particularly alkaline cells intended for a range of normal service including high power applications.